Quantum Mechanics: Postulates


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1 Quantum Mechanics: Postulates 5th April 2010 I. Physical meaning of the Wavefunction Postulate 1: The wavefunction attempts to describe a quantum mechanical entity (photon, electron, xray, etc.) through its spatial location and time dependence, i.e. the wavefunction is in the most general sense dependent on time and space: Ψ = Ψ(x, t) The state of a quantum mechanical system is completely specified by the wavefunction Ψ(x, t). The Probability that a particle will be found at time t 0 in a spatial interval of width dx centered about x 0 is determined by the wavefunction as: P (x 0, t 0 ) dx = Ψ (x 0, t 0 )Ψ(x 0, t 0 )dx = Ψ(x 0, t 0 ) 2 dx Note: Unlike for a classical wave, with a welldefined amplitude (as discussed earlier), the Ψ(x, t) amplitude is not ascribed a meaning. Note: Since the postulate of the probability is defined through the use of a complex conjugate, Ψ, it is accepted that the wavefunction is a complexvalued entity. Note: Since the wavefunction is squared to obtain the probability, the wavefunction itself can be complex and/or negative. This still leaves a probability of zero to one. Note: Ψ is the complex conjugate of Ψ. For instance: 1
2 Ψ(x) = A e i k x Ψ (x) = (A ) e i k x Since the probability of a particle being somewhere in space is unity, the integration of the wavefunction over all space leads to a probability of 1. That is, the wavefunction is normalized: Ψ (x, t)ψ(x, t)dx = 1 In order for Ψ(x, t) to represent a viable physical state, certain conditions are required: 1. The wavefunction must be a singlevalued function of the spatial coordinates. (single probability for being in a given spatial interval) 2. The first derivative of the wavefunction must be continuous so that the second derivative exists in order to satisfiy the Schrödinger equation. 3. The wavefunction cannot have an infinite amplitude over a finite interval. This would preclude normalization over the interval. II. Experimental Observables Correspond to Quantum Mechanical Operators Postulate 2: For every measurable property of the system in classical mechanics such as position, momentum, and energy, there exists a corresponding operator in quantum mechanics. An experiment in the lab to measure a value for such an observable is simulated in theory by operating on the wavefunction of the system with the corresponding operator. Note: Quantum mechanical operators are clasified as Hermitian operators as they are analogs of Hermitian matrices, that are defined as having only real eigenvalues. Also, the eigenfunctions of Hermitian operators are orthogonal. Table 14.1 (Engel and Reid): list of classical observables and q.m. operator. 2
3 Observable Operator Symbol of Operator Momentum Kinetic Energy h 2 2m i h x 2 x 2 ˆp x Ê kinetic Position x ˆx Potential Energy V (x) ˆV h Total Energy 2 2 2m x + V (x) Ĥ 2 Note: operators act on a wavefunction from the left, and the order of operations is important (much as in the case of multiplying by matrices commutativity is important). III. Individual Measurements Postulate 3: For a single measurement of an observable corresponding to a quantum mechanical operator, only values that are eigenvalues of the operator will be measured. If measuring energy: one obtains eigenvalues of the timeindependent Schrödinger equation: ĤΨ n (x, t) = E n Ψ n (x, t) Note: The total wavefunction defining a given state of a particle need not be an eigenfunction of the operator (but one can expand the wavefunction in terms of the eigenfunctions of the operator as a complete basis). IV. Expectation Values and Collapse of the Wavefunction Postulate 4: The average, or expectation, value of an observable corresponding to a quantum mechanical operator is given by: < a >= Ψ (x, t)âψ(x, t)dx Ψ (x, t)ψ(x, t)dx This is a general form for the expectation value expression. If the wavefunction is normalized, then the denominator is identically 1 (this is assumed to be the case since every valid wavefunction must be normalized). Consider the following cases: 3
4 A. The wavefunction represents an eigenfuntion of the operator of interest. The expectation value of a measurable is: < a >= φ j (x, t)âφ j(x, t)dx = a j φ j (x, t)φ j(x, t)dx where φ j (x, t) is an eigenfunction of the operator Â. B. Now consider that the wavefunction itself is not an eigenfunction of the operator Â. However, since the wavefunction belongs to the space of functions that are eigenfunctions of the operator, we can construct (expand) the wavefunction from the basis of eigenfunctions of Â. Thus: Ψ(x, t) = b n φ n (x, t) where the φ n (x, t) are eigenfunctions of the operator Â. So let s consider the = < a >= [ φ j(x, t)âφ j(x, t)dx ] [ b m φ m (x, t) b n a n φ n (x, t) m=1 n=1 ] dx = n=1 m=1 b mφ m(x, t)b n a n φ n (x, t)dx = n=1 m=1 b m φ m (x, t)b na n φ n (x, t)dx The only terms that survive are the m=n terms (orthogonality of eigenfunctions of Hermitian operators!). Thus, < a >= b nb n a n n=1 4
5 = b n 2 a n n=1 Note: We see that the average value of the observable is a weighted average of the possible eigenvalues (i.e., possible measurement outcomes). The weighting factor is the square of the expansion coefficient, b n for the n th eigenfunction with eigenvalue a n. Thus, the expansion coefficients, b n represent the degree to which the full wavefunction possesses the character of the eigenfunction φ n. By anology to vector space, the coefficients can be thought of as projections of the wavefunction on to the basis functions (i.e., the eigenfunctions of the operator). Note: Individual measurements of a quantum mechanical system have multiple outcomes. We cannot know which of these will be observed a priori. We only know in a probabilistic way the relative opportunities to realize any particular value. Furthermore, the probabilities become meaningful in the limit of large numbers of measurements. Note: identically prepared quantum systems can have different outcomes with regard to measure observables. (super position of states) Note: Initial measurement of a quantum mechanical system is probabilistic. Subsequent measurements will yield the same value of the observable. This is a transition from a probabilistic to a deterministic outcome. Collapse of the wavefunction. V. Time Evolution Postulate 5: The timedependent Schrödinger equation governs the time evolution of a quantum mechanical system: Ψ(x, t) ĤΨ(x, t) = i h t Note: The Hamiltonian operator Ĥ contains the kinetic and potential operators (as discussed above). This equation reflects the deterministic (Newtonian) nature of particles/waves. It appears to be in contrast to Postulate 4 (many observations lead to different measured observables, each weighted differently, i.e., a probabilistic view of the particle/wave). The reconciliation is in the fact that Postulate 4 pertains to the outcomes of measurements at 5
6 a specific instant in time. Postulate 5 allows us to propagate the wavefunction in time (we propagate a probabilistic entity). Then, at some future time, if we make another measurement, we are again faced with the implications of Postulate 4. Note: In dealing stationary quantum mechanical states, we do not need to have an explicit knowledge of the time dependent wavefunction, particularly if the operator of interest is independent of time. Consider: Â(x)Ψ n (x, t) = a n Ψ n (x, t) Recalling that the time dependent wavefunction can be written as the product of timeindependent and timedependent components: Thus: Ψ n (x, t) = ψ n (x)e i(en/ h)t Â(x)ψ n (x)e i(en/ h)t = e i(en/ h)t Â(x)ψ n (x) = e i(en/ h)t a n ψ n (x) Â(x)ψ n (x) = a n ψ n (x) Thus, for timeindependent operators, the eigenvalue equations for ψ n (x) are all we need to consider. 6
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